Process for manufacturing steel

ABSTRACT

Process for manufacturing steel, particularly special and alloyed, in a vessel revolving around its vertical axis, furnished with means for varying sharply its velocity or reversing the sense of rotation, said vessel being internally lined with basic or acid refractory, the steelmaking being carried out by downwardly blowing oxydizing gases, in particular technically pure oxygen, over the melt consisting of liquid pig iron, iron or steel scarp and/or iron in pigs, flux and possibly iron ore, said melt being dynamically curved by the rotation; in which process the oxygen required for the refining, or other oxydizing or inert gases are blown downwardly, these being called primary jet, and other oxygen and/or other oxydizing gases, called secondary jet, are blown down in order to burn up to 90 percent of CO formed during the refining actions and form CO2 inside the vessel, including the metallic melt is directly overheated by the heat developed by said combustion of CO, without using other fuels, being the refractory lining of the vessel protected against the action of the heat as well as of the slag during the refining.

United States Patent 1191 Ramacciotti 45 F b, 19, 1974 [54] PROCESS FOR MANUFACTURING STEEL 979,605 1/1965 Great Britain 75/60 1751 Inventor Aldo Ramacciom, Rome, My 13123323 351323 8123 322222 3::::...,...........:::::: 32/28 [73] Assignee: Centro Sperimentale Metallurgico S. .A. R It I p ome a y Primary Examiner-Hyland Bizot Flledl y 1971 Assistant ExaminerPeter D. Rosenberg [21] APPL NOJ 140,962 Attorney, Agent, or Firm-Yung & Thompson Foreign Application Priority Data T May 22, 1970 ltaly 50894/ [57] ABS RACT 52 us. (:1. /60, 75/61 Pmess for Yanufacmrmg i i' and alloyed, in a vessel revolvmg around 1ts vertical [51] Int. Cl. C2lc 5/32 axis furnished with means for varyin Sharply its [58] Field of Search 75/61, 60, 59, 51, 52, 46,

75] locity or reversing the sense of rotatlon, sa1d vessel being internally lined with basic or acid refractory, the [56] Reierences Cited steelmaking being earned out by downwardly blowmg oxydizmg gases, 1n partlcular techmcally pure oxygen, UNITED STATES PATENTS over the melt consisting of liquid pig iron, iron or steel 3,689,250 9/1972 Roeder 75/11 l-p and/0r iron in pigs, flux and possibly iron ore, 2,828,516 4/1953 Black et 266/37 said melt being dynamically curved by the rotation; in 3 3 3 11/1960 75/60 which process the oxygen required for the refining, or l555 1/1962 l' 75/60 other oxydizing or inert gases are blown downwardly, 3,672,869 6/1972 N ehaus 75/60 th b n d t d th I d 2,862,811 l2/l958 Eketorp etal... 75/60 ese emg ca 1e 0 2,959,479 11/1960 Graef 75/60 ("her OXYdZmg gases Called secondaiy are 3,271,128 9/1966 Tartaron 75/60 blown down in Order to burn "P to Percent Of C0 2,978,318 4/l96l Kalling et al 75/60 x f d du g th efining actions and f rm C0 3,313,619 4/1967 Decamps 75/60 inside the vessel, including the metallic melt is directly 3,034,335 5/I962 Hard! 75/52 overheated by the heat developed by said combustion 2,977,217 3/1961 Graef 6t 8| 75/60 of CO, without using other fuels being the refractory 312511681 5/1966 wakamats at 75/61 lining of the vessel protected against the action of the 3,401,034 9/1968 MOOI'B 75/61 heat as we" as of the g during the refining- FOREIGN PATENTS OR APPLICATIONS 943,267 12/1963 Great Britain 75/60 1 Claim, No Drawings PROCESS FOR MANUFACTURING STEEL The present invention relates to an improved process for manufacturing steel. More particularly, the object of this invention is a process wherein the charge material (liquid pig iron, with scrap,slag, flux and if the case may be also iron ore) is contained :in-a vertical symmetrical vessel, revolving at a suitable speed and sense about its vertical axis and will be refined by blowing onto its surface, by means of suitable devices, oxidizing refining gas, particularly technically pure oxygen.

As it is known from the prior art, the making of non common and alloyed steel requires the recourse to relatively long processes, the use of particularly selected charges (steelmaking in electric furnace) or of cumbersome, space requiring and expensive plants having a low yield (Martin-Siemens furnaces),

Other processes at present used, like those known under the names of LD (or BOF) and LD-AC allow the treatment times to be remarkably shortened with respect to the previous art and particularly with respect to the conventional Bessemer and Thomas converters, and require relatively simple plants; however by these processes steels of a normal quality are produced, and high amounts of scraps cannot be used. lnorder to .obvia-te this drawback it has been proposed to pre-heat the scrap within or outside the converter, but this operation requires pre-heating plants which are rather expensive and complicated, or the use. of silicon carbide and/or calcium carbide, with a consequent increase of the manufacturing cost. 1

Another technical proposed solution consists in in creasing the temperature inside the converter by burning therein suitable fuels, particularly the carbon monoxide which is formed during the refining process.

However, in a stationaryuconverte-r, most of the re.- fractory is exposed to the intense heat generated by this combustion of CO, and'th-us it will be worn in a very short time. 1

Thereby the need derives of getting rid of the heat absorbed in a. refractory in a useful way; so was inventedthe furnace revolving about a horizontal or subhorizontal axis, wherein the area of the refractory'lining which is not submerged is over-heated by the combustion of carbon oxide and then, due to its revolution, it contacts the metal bath and delivers thereto part of the absorbed heat. This type of process, although rather spread, shows severe drawbacks due to wear of the refractory as the whole surface thereof is exposed in sequential times, to the damages caused byv heat,.to the attack of the slag andto the erosion due to the trio-- tion between the metal bath, which is almost stationary, and the lining which is revolving. These, reasons render the average life of the refractory lining of these furnaces somewhat lower than'that of a conventional LD converter.

Several attempts have been made in order to diminish the damage of the refractory, but none of them reached the aimed purpose. For instance the French Pat. No. 1,207,464 describes a converterrevolving at high speed about its vertical axis, provided with spout hole through its bottom and further provided with a lance blowing on the bath surface, dynamicallydisposed against the vertical walls of the converter, a refining gas, through a set of nozzles distributed along a generatrix of the lance itself located. parallel to the bath. The jets of the so blown gas will cause an intense rabbling of the metal with theslag, and further hit an ever new surface, thus reducing the local temperature and the formation of red smokes.

On the ground of tests carried out with revolving converters,-it has been noted that if one part of the bottom of the converter is left uncovered and exposed to the direct action of the heat and,"in an annular zone also to the slag attack, as in fact it occurs in said French Patent, the other advantages as aforecited will be reduced to a determinant amount; in fact the part of refractory left uncovered will undergo a quick physical degradation, so that after a very reduced number of heats, the work must be discontinued and the refractory lining must be restored. Also the high speeds of revolution up to 500 rev./m'in., as disclosed in said French Patent do not appear to be the most suitable for an industrial-type converter. In said Patent, also, no word is spent for the possibility of over-heating the bath by supply'of a heat different from that generated by the refining reactions.

It is evident, therefore, that up to now no process has been actuated conciliating the need of using installations which are capable of operating for a long time requiring no relining, with the need of making high grade steels in very short times and at a lower cost.

According to the present invention it is possible to overcome the above cited drawbacks concerning the refractory consumption, by providing a process whereby it is possibleto make special steels in times comparable to those of the LD process, but with. a greater production capacity at parity of internal vol 'ume of the converter, and therefore at a lower cost.

The process forming the object of the present invention consists in refining'a charge including liquid pig iron, steel scrap and/or iron in pigs, flux and possiblyiron ore and/or other cooling material by blowing one.

or more streams of refining oxygen or other oxidizing gas, mixed, if the case may be, withinert gas (argon, ni-

trogen and the like) in a proportion variable up to the.

wherein the blowing of said primary jet and of said sec-.

ondary jet ensures a quick development of the refining reactions and therefore an effective overheating of the bath, the refractory lining 'being protected against the heat action during the refining.'This is possible insofar as the revolution of the converter, and therefore of the molten bath, determines a distribution of a part of the I molten mass'of the bath against almost the entire vertical wall of the converter, screening it against the intense heat generated during said combustion action, the balance remaining to protect the bottom. Furthermore, according to the present invention, this rise of the bath along the walls of the converter is utilized in order so to increase the amount of the charge, so that the ratio: total converter inner volume/weight of the charge, is equal at the utmost to 0.4 m lt versus a ratio of about 0.8 to l m /t which can be obtained in a vertical stationary converter. It is to be noted that this increase of charge does not prevent the refining times from being kept within the limits of a conventional LD process; in fact due to revolution, the thickness of the bath will be at any point suitable for a quick and effective refining. The speed of revolution of the converter appears to'be rather critical-to the purposes of both the metallurgical process and the protection of the refractory. The thickness of the bath, in fact, must be such as to prevent the primary jet from reaching the lining of the converter and at same time must be not so great as to promote the generation of dead zones little interested or not interested to the refining reactions. It has been found that preferably the revolution speed must be such as to allow the formation of a paraboloid the height of which stands in a relation with the internal diameter of the converter according to a ratio comprised between 0.7 and 1.5.

Furthermore, to avoid the wear of the refractory due to the friction between the latter and the bath, according to the present invention, the speed of revolution of the converter must be established in relation to the dimensions of the converter itself, so that at the rated speed, the peripheral speeds of the basic or acid refractory and of the metal bath, at the separating surfaces thereof will be equal, avoiding, as aforesaid, that wear which hindered a large diffusion of those processes, for instance the Kaldo process, where said relative movements exist.

Another feature of the present invention, connected to the primary blowing, consists in that the slag is kept apart from the basic or acid lining so that said slag cannot attack the lining. In fact the slag which is lighter than the liquid metal, will locate on the surface of the paraboloid generated by. the roation of the bath and tends to rise, heaping near the upper end of said paraboloid. This fact would involve the double drawback of wearing quickly the refractory zone concerned with the heaping of the slag, and of leaving the bath at the point or points of impact of the oxygen and/or the oxidizing gas, free from slag, slowing down the refining process. These drawbacks will be overcome, according to the present invention, by directing the primary jet so as to maintain utmost of the slag at the apex of the paraboloid, the remaining part being located on the bath in the form of a thin layer and/or of foam.

It has been found that this purpose and together that of an optimum development of the refining reactions,

can be attained according to this invention, by directing the primary jet so as to hit the bath at a characteristic position, variable according to the form and capacity of the vessel; particularly it has been found that for the industrial converters, the impact point of the primary jet onto the surface of the paraboloid must be at a distance from the axis of the converter, measured on the radius of the cross section-of the converter at that point, comprised between percent and 40 percent of the value of the internal diameter of the converter itself.

Due to this particular blowing, certain streams will be formed on the surface of the paraboloid and above said impact point, said streams being directed towards the impact point itself and hindering the upwards movement of the slag due to centrifugal force; under this impact point the strength of the jet itself keeps the slag gathered at the apex of the paraboloid, the speed of rotation being such as to ensure that each point of the'paraboloid come to be located under the primary jet before the slag had been capable of climbing remarkably beyond said impact point. Thus an effective rabbling of the slag with the molten bath will be obtained with a consequent improvement of the refining.

According to the present invention, furthermore, the blowing of the primary jet is carried out by means of a lance with a vertical single nozzle which is eccentrically located and adjustable both vertically and horizontally, the eccentricity of which is comprised between 10 percent and 40 percent of the value of the internal diameter of the converter, said lance being so located that its head will be at a distance from the impact point comprised between 20 percent and 65 percent of the value of the internal diameter of the converter.

In a further embodiment of the process according to this invention, the blowing is carried out through a lance located at the centre, along the vertical axis of the converter; in this case'said lance is provided with one or more nozzles, downwards directed with their axes inclined with respect to the axis of the lance itself through an angle from 15 to 45. This-lance is vertically adjustable and its distance measured along the axis of the nozzles, from said nozzles to the impact point of said jets on the surface of the bath, equals a value from 18 percent to 65 percent of the value of the internal diameter of the converter.

Another feature of the present invention consists in that the heat developed by the combustion of CO to CO is absorbed directly by the metal bath, rotated by means known per se, as disclosed in the first path of this specification. In fact according to this invention, an amount up to percent of said CO will be effectively burnt, utilizing the combustion heat in order to melt amounts of scrap from 20 percent to more than 50 percent of the total charge and/or corresponding amounts of pig iron in pigs and/or iron ores, with no need of preheating or of the addition of other fuel. According to theamount of cold charge it is possible to utilize said combustion heat also for overheating the metal bath, so as to be in a position as to carry out, at the end of the refining operation, further metallurgical operations in order to obtain a high grade final product.

The blowing of the secondary jet is embodied according to the present invention so that this secondary jet hit directly neither the surface of the meta] bath nor that of the refractory; particularly, said secondary jet is delivered through nozzles the diameter of which varies from 1/50 to 1/150 of the distance measured along the axis of the nozzles themselves, from said nozzles to the molten bath.

1f the primary jet is blown by an eccentric lance, the blowing of the secondary jet can be obtained by means of a second lance vertically and horizontally adjustable, the eccentricity of said second lance varying from nil to the value of the eccentricity of the primary lance, said second lance having one or more nozzles and being introduced into the converter so that said nozzles will be at a depth, measured from the mouth of the vessel, from 10 percent to 40 percent of the value of the internal diameter of the converter.

.If the blowing of the primary jet is effected by means I of a lance located along the axis of the converter, the

secondary jet will be blown by auxiliary nozzles, suitably located along the circumference of the head of the lance spaced apart from the endless than two diameters of the lance, or along the circumference of the lance body..ln this second case the auxiliary nozzles are downwards turned and are inclined with respect to the axis of the lance through an angle from 10 to 30; their position inside the converter is soselected as to establish their distance from the plane of the mouth at a value from percent to 40 percent of the internal diameter of the converter itself. If the main lance is located along the axis of the converter the blowing of the secondary jet can also be made of a second lance provided with one or more nozzles, located ecce-ntrically and horizontally or vertically'adjustable, with an eccentricity variable from 10 percent to 40 percent of the value of the internal diameter of the converter. This second lance is introduced into the converter so that said nozzles will be located at a variable depth measured from the mouth of the converter; the maximum effect will be reached at a depth from 10 percent to 40 percent of the internal diameter of the converter. The rate of this secondary jet can vary from 40 percent to 100 percent of the value of the rate of the primary jet, said secondary rate being adjustable, independently from that of the primary jet, during the process, from zero to the optimum value. 1

The over-heating caused by the combustion of CO is such as to render useless the addition of slag fluidizing substances, and furthermore it-is capable of allowing,

at the end of refining, subsequent operations of slagreplacement, of purifying and of alloying: thus, in a short while and with no subsequent supplies of heat a high grade product will be obtained, which could not be obtained by processes having an equal duration, and 'com parable to, or higher than that which can be obtained in far longer times with the electric furance. Moreover, the, aforesaid fact that part of the surface of the bath is covered only by a thin layer of slag will render quicker and more effective the overheating of the bath itself.

The process according to this invention results to be particularly flexible; it can be utilized not only in those plants where scarce amounts of liquid pig ioron are available, but also in plants where the liquid pig iron contains high amounts of sulphurand phosphorus as well as of other oxidizable impurities. Also in this case the process according to this invention is capable of by any other process, in correspondingly short times.

In order to better illustrate the advantages of the present invention, hereinafter there are: a Table (Table l summarizing the data of certain heats effected, some examples of process and a Table (Table 2) showing the refractory consumption, wherein by way of example some average attained results are shown together with some preferred types of process, according to the present invention. 5

Table 1 shows the data of certain heats, selected in the groups of heats carried out for testing the various type of blowing.

Column 1 shows the data of one of thefirst heats carried out to ascertain thevalidity of the process, and wherein only primary oxygen has been blown.

Columns 2, 3, 4 and 5 show the data of heats effected by blowing also secondary oxygen. Two lances have been used with variable excentricity and penetration inside the converter.

Columns 6, 7 and 8 show the data of casting effected by blowing also secondary oxygen and utilizing a single lance capable of delivering, from different nozzles, primary and secondary oxygen.

In the columns 1, 2, 3, 5 and 7 possible additions of iron alloys were subsequently effected in the ladder.

In the castings 4, 6 and 8 the Fe-Si has been added in the conveter.

EXAMPLE 1 This example relates to the heat reported in Table l 7 under N0. 5. In this heatthe effectiveness of the process has been tested under the standpoint of the refining as well as of the final yield. The sequence of the operations has been as follows:

1. Charging part of the lime (about two-fifths of the total), then scrap, liquid pig iron, and lastly other twofifths of lime, in the order;

2. starting the converter rotating;

3. lowering the main lance and low-pressure blowing;

4. stabilizing simultaneously the speed of revolution at the rated value;

5. mixing for de-phosphorizing; replacing the slag;

6. putting into operation the main lance and the main O blowing has reached its rated value;

7. starting of the decarburization and addition of the last amount of line;

8. positioning of the secondary lance and starting blowing the secondary O 9. blowing of secondary 0 at its rated value;

10. removing the secondary lance; rating the secondary O to nil;

l l. discontinuing the main O blowing;

l2. simultaneously removing the primarylance and starting stopping the converter;

l3. short rabbling;

l4. concerter stopped; tilting of the converter, temperature measurement and bath and slag sampling;

l5. inclining of the converter and tapping.

During the test there was a slight slopping during the first minutes of blowing.

The primary oxygen was blown by an eccentric lance with one nozzle and the secondary oxygen was blown by a lance also eccentric and with two nozzles having a 32 mm diameter. The percent ratio, between the rate of secondary O and the rate of the primary 0 was equal to 100.

As is shown in the Table, the ratio between the final sulphur and the initial sulphur is 0.33 and the ratio between the final phosphorus and'the initial phosphorus is about 0.062. g

The oxygen and nitrogen content of the steel resulted to be less than 0.001 percent. The iron yield was 93.6 percent.

A rabbling cycle consists in starting the converter and causing it to reach the rated speed, then stopping the converter, and subsequently starting it in opposite This example relates to the heat reported in Table 1, under N0. 6.

This heat, one of the first heat effected with a single lance and delivery of primary and secondary 0 was directed to ascertain the capacity of desulphuri'zation. To this purpose of liquid pig iron with a very high sulphur content and a little amount of scrap has been charged. The operative sequence has been identical to that shown in Example 1, apart from the fact that the rabbling and the slag replacement according to point 5 have not been carried out, and after the operation 12, the following operations have been carried out:

8 The final sulphur/initial initial. sulphur ratio was 0.089, while no de-phosphorization operation being carried out the ratio of the final phosphorus to initial 9 393 99525 wa The oxygen content of the final steel was 0.0091 and the nitrogen content was 0.010. 1

Owing to the relatively small capacity of the used converter, and due to a remarkable slopping and of the various slag replacing operations. the iron yield has been 88.5 percent.

The Table 2 shows the refractory consumption alter 50 non continuous heats.

ter was 0.6 percent of the distance from said nozzles to the metal bath.

As the refractory was a dolomite ramming, its consumption resulted lower than that usually obtained.

TABLE] v Heat No. 1 2 3 4 5 6 7 Percent composition of the charge:

Liquid pig iron 83 74 65 83 71 83 56 80 Scrap........... 13 22 30 13 13 40 14 Lime 4 4 5 V 4 4 4 6 Percent composition of the liquid pig iron:

3.6 3.5 3.6 3.6 3.6 3.32 3.6 3.4 0.49 h .75 7 "A 1.01 A 7 0.56 0.62 0.62 A 0.48 0.84 0.64 0.85 0.84 0.87 0.72 0.64 0.61 0.70 0.10 0.11 0.15 0.13 0.16 0.16 0.09 0.11 0.032 0 057 0.054 0.045 0.079 0.112 0.063 0.033 1405 1370 1364 1370 1366 1370 1373 1371 Primary blowing characteristics:

EccentricityLconv. diam. (percent).. I 20 15 20 25 Height/conv. diam. (percent) 25 20 25 25 25 40 60 Secondary blowing characteristics:

Eccentricity/conv. diam. (percent). 0 I 10 20 20 Depthlconv. diam. (percent) 15 20 30 20 25 15 No. of nozzles l 4 2 2 2 6 4 6 Nozzles diam/bath dist. (percent) 0.6 3.2 3.2 3.2 0.4 0.6 0.8 Sec. rate Oe/Plll'fl. rate Q (percent) r 61 100 100 100 100 77 100 Ratio of comb. burnt CO/produced CO (percent) 41 54 64 52 77 43 38 0 total consumption Nm /t steel... 51 44 55 58 49 53 46 43 Blowing duration min 19 15 18 15 22 15 14 18 Slagging and subsequent rabblings:

N0. of cycles 3 6 4 4 Blow. duration/work duration (percent) 53 6 52 52 Final composition of steel when tapped (percent):

0.14 0.16 0.08 0.40 0.16 0.37 0.09 0.50 0.28 0.29 0.09 0.19 0.14 0.35 0.13 0.22 traces traces traces 0.25 traces 0.17 traces 0.26 0.022 0.021 0.005 0.029 0.010 0.053 0.009 0.034 0.022 0.035 0.016 0.004 0.026 0.01 0.026 0.026 0.0065 0.0091 0.010 0.0080 1660 1625 I590 1632 1615 1621 1620 1646 lron yield 94.8 95.2 93.4 94.0 93.6 88.5 92.9 94.7 Slag characteristics at the end point:

Total Fe 17.2 12.5 26.1 20.0 21.6 10.5 23.1 14.2 Basicity index 3.4 3.5 5.2 5.0 4.7 2.6 4.8 3.3

TABLE 2 e Distance from mouth/total useful height of the c onverter, (perc e nt) 7 s.

. as". .L 5 m... l 0 7 11 14 17 21 25 29 32 36 39 43 46+65 68 +74. 77+100 w mm ii se 5f erdil t grylts this st t jssstnsrscptp,

32.5 25 22 i 25 V v 24 7 20 20 T8 15 "E RY 0 1 0 axis in a direction to urge said slag toward said axis and away from said refractory lining, said distance being 10 to 40 percent of the inner diameter of the vessel, the

speed of rotation of the vessel being such that said paraboloidal surface has a height whose ratio to the internal diameter of the vessel is between 0.7 and 1.5, and the filling ratio of the vessel, expressed as the ratio between the total inner volume of the vessel in cubic meters and the charge weight in metric tons, being below 0.4. 

